Why does a person who is a student not have a deferment in reserve+? ...

The mere fact of studying full-time or dually does not automatically mean that the student has a deferment.

To get a postponement, you need to contact the territorial procurement center (TCC), submit an application and supporting documents. They include an extract from the enrollment order and a certificate from the educational institution. The TCC, in turn, can verify the fact of training by sending a corresponding request to the educational institution.

Also, in order to receive a deferment, you will most likely need to go through such stages as updating credentials and passing a military medical board.